I'm not familiar with the details of Nikon AF, but certainly every camera I've owned often finds the background more interesting than the subject at times.

If you're using a multipoint AF then make sure it has lock on the subject as it tracks. If you're good at tracking the subject yourself, I find single point AF to work quite well. Obviously that is limited more to subjects that fly more predictably or slowly. If they're in the same kind of area all the time, manual pre-focus can be useful too.

What shutter speeds are you using? A fast shutter helps to prevent motion blur.

Conversely, you might want to stop down the lens a bit. I find the DoF can be very shallow, and increasing the DoF can help cover any focus errors.

Conversely, you might want to stop down the lens a bit. I find the DoF can be very shallow, and increasing the DoF can help cover any focus errors.

I try to shoot at around f/8 for bif for the reason popo stated above.
You may very well have to crank up your iso to get your shutter speeds up.

Try using 21-point AF, and for myself I avoid using a tripod for all my wildlife shooting as to me anyway, is a pain in the ass and have missed some great photo opps.

Not sure what birds you intend on shooting, but the 70-200 in most cases you will find too short. Unless you are after local ducks and gulls.

For the money you would spend on the Tamron 70-200 I would instead look at the Sigma 120-400 or the 150-500. Not fast lenses but you will get the reach. For the money the Nikon afs70-300vr is a great lense.

Practice on your panning very important, try moving your hips only to pan not your arms or upper body.

Used AF-C so it doesn't lock the focus and keeps tracking the subject.

I don't think I'm going to need to bump up the ISO's just because there is a lot of light outside, but I will surely try up to about 500 next time I go out.

Do you have any experience with the Sigma lenses quality wise? I'm all for saving money but at the same time I don't want to buy a lens that I'm going to regret because the Optics and build quality is crap.

AF-C with Dynamic AF-Area mode and the camera set to sports mode and continuous shutter release.

The other thing is the autofocus will track whatever is in the centre of the focus point when you first hold down the shutter button (half way) so if your subject is not within the focus point and your background is you track the background.

I'm quite happy with results from both my Sigma's.
They aren't overly expensive which is great for me and they seem to produce well enough for my liking at this time.
I have no experience with better lenses (yet) so have no comparison.
You can have a look at some of my stuff throughout the wildlife forum here,the 500mm is almost always attached to my cam...
Whitetail deer
Birds
Duck of the day
Marine mammals

I "try" to do as little post processing as possible so what you see for the most part is what comes from both the camera and lens.
The 300mm is a good start but I have found the 500 to take a better pic.
I got the 300 (less than $300) to start before jumping up just to know if I would get the use from the 500 ($1200) to justify the purchase.

I'm by far no pro and I learn something new every time out and each time I come on Cameralabs.
I think maybe I haven't drawn the full potential from this lens yet but I can certainly say I've got my monies worth from it